Java and /etc/resolv.conf DNS server failover

domain-name-systemjava

Executive Summary

Does Java query the next resolv.conf name server entry if the first one fails or times out?
What is the default timeout at which Java will consider a resolver "down"?

Details

In /etc/resolv.conf, in most sites, there are multiple DNS servers configured for use by the system. Can anyone clarify what happens, with Java, if the first DNS server in the list fails? For example, imagine the following resolv.conf:

nameserver 8.8.8.8
nameserver 4.4.4.4

If 8.8.8.8 becomes unreachable then will Java try 4.4.4.4? In our experience it does not – we saw many threads waiting to resolve DNS names even though the backup DNS servers were reachable.

Best Answer

On Linux java will delegate hostname resolution to the underlying C system library which does the actual resolving.

The name servers will be queried in the order listed as specified in /etc/resolv.conf after the other sources of information specified in /etc/nsswitch.conf have been queried (think /etc/hosts but possibly NIS or LDAP).

You can specify a timeout value in /etc/resolv.conf , according to the man page the default is 5 seconds.

If your first server gives a valid error code I.e. servfail the second name server will not be queried

Related Solutions

How to add extra DNS server to handle the own TLD domains

In order to add a new TLD that ICANN doesn't yet recognize (like you are) you have to put a DNS server that considers itself authoritative in the DNS resolution chain. In practice, this means the first hop. In order to use your *.top domain internally, you will have to either point all of your clients at your own DNS server, or maintain /etc/hosts files. One of the two.

As for top.my-domain.com, you have some options. Whichever DNS servers are authoritative for my-domain.com (you don't say) will be able to also serve up top.my-domain.com. This may be your best bet for simplicity's sake. Have your (presumed) DNS hosting service add that subdomain and add the entries you need.

However, hosting a local DNS server is not a bad idea at all really. They're called a Caching DNS server and cache resolved names so they can be retrieved faster than they would be pulling from the Internet. If you add your *.top domain to it, it'll provide all of your needs.

How to configure the timeout that ping uses to resolve a hostname with first and second name servers

The correct syntax is:

nameserver 10.10.1.15
nameserver 10.10.1.16
options timeout:1 rotate attempts:1